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DEFECTS IN FLEXIBLE PAVEMENT Prepared By

Umar JanYounis Farooq Types of pavement deterioration:

The four major categories of common asphalt pavement surface distresses are:

Cracking

2. Surface deformation 3. Disintegration (potholes, etc.)

4. Surface defects (bleeding, etc.) DEFECTS IN FLEXIBLE PAVEMENTSDefects inflexible pavements can be placed into one of five classes. These classes are cracking, distortion, disintegration, slippery surfaces, and surface treatment problems.A)CRACKING : ALLIGATOR CRACKS.- Alligator cracks are interconnected cracks, forming a series of small blocks resembling an alligator's skin or chicken wire. In most cases, alligator cracking is caused by excessive movement of the surface over unstable subgrades or base courses. The unstable support is the result of saturated granular bases or subgrade. Normally, the affected area is not large. When it doesoccur on a large scale, the cracking is most likely due torepeated loads above the designed strength of the pavement.

2. Longitudinal cracking: Longitudinal cracks are long cracks that run parallel to the center line of the roadway. These may be caused by frost heaving or joint failures, or they may be load induced. Understanding the cause is critical to selecting the proper repair. Multiple parallel cracks may eventually form from the initial crack. This phenomenon, known as deterioration, is usually a sign that crack repairs are not the proper solution.

3. Reflective cracking: Reflective cracking occurs when a pavement is overlaid with hot mix asphalt concrete and cracks reflect up through the new surface. It is called reflective cracking because it reflects the crack pattern of the pavement structure below. As expected from the name, reflective cracks are actually covered over cracks reappearing in the surface. They can be repaired in similar techniques to the other cracking noted above. Before placing any overlays or wearing courses, cracks should be properly repaired.

B. Surface deformation: Pavement deformation is the result of weakness in one or more layers of the pavement that has experienced movement after construction. The deformation may be accompanied by cracking. Surface distortions can be a traffic hazard. The basic types of surface deformation are: 1. Rutting 2. Corrugations 3. Depressions 1. Rutting Rutting is the displacement of pavement material that creates channels in the wheel path. Very severe rutting will actually hold water in the rut. Rutting is usually a failure in one or more layers in the pavement. The width of the rut is a sign of which layer has failed. A very narrow rut is usually a surface failure, while a wide one is indicative of a subgrade failure. Inadequate compaction can lead to rutting. Figure shows an example of rutting due to subgrade Failure. Minor surface rutting can be filled with micro paving or paver-placed surface treatments.

2. Corrugation Corrugation is referred to as wash boarding because the pavement surface has become distorted like a washboard. The instability of the asphalt concrete surface course may be caused by too much asphalt cement, too much fine aggregate, or rounded or smooth textured coarse aggregate. Corrugations usually occur at places where vehicles accelerate or decelerate. Minor corrugations can be repaired with an overlay or surface milling. Severe corrugations require a deeper milling before resurfacing.

3. Depressions Depressions are small, localized bowl-shaped areas that may include cracking. Depressions cause roughness, are a hazard to motorists, and allow water to collect. Depressions are typically caused by localized consolidation or movement of the supporting layers beneath the surface course due to instability. Repair by excavating and rebuilding the localized depressions. Reconstruction is required for extensive depressions.

C. Disintegration The progressive breaking up of the pavement into small, loose pieces is called disintegration. If the disintegration is not repaired in its early stages, complete reconstruction of the pavement may be needed. The two most common types of disintegration are: 1. Potholes 2. Patches 1. Potholes Potholes are bowl-shaped holes similar to depressions. They are a progressive failure. First, small fragments of the top layer are dislodged. Over time, the distress will progress downward into the lower layers of the pavement. Potholes are often located in areas of poor drainage, as seen in Figure. Potholes are formed when the pavement disintegrates under traffic loading, due to inadequate strength in one or more layers of the pavement, usually accompanied by the presence of water.

2. Patches:

A patch is defined as a portion of the pavement that has been removed and replaced. Patches are usually used to repair defects in a pavement or to cover a utility trench. Patch failure can lead to a more widespread failure of the surrounding pavement. Some people do not consider patches as a pavement defect. While this should be true for high quality patches as is done in a semi permanent patch, the throw and roll patch is just a cover. The underlying cause is still under the pothole.

D. Surface defects: Surface defects are related to problems in the surface layer. The most common types of surface distress are: 1. Raveling 2. Bleeding 3. Polishing

Raveling: Raveling is the loss of material from the pavement surface. It is a result of insufficient adhesion between the asphalt cement and the aggregate. Initially, fine aggregate breaks loose and leaves small, rough patches in the surface of the pavement. As the disintegration continues, larger aggregate breaks loose, leaving rougher surfaces. Raveling can be accelerated by traffic and freezing weather. Some raveling in chip seals is due to improper construction technique. This can also lead to bleeding. Repair the problem with a wearing course or an overlay.

2. Bleeding:

Bleeding is defined as the presence of excess asphalt on the road surface which creates patches of asphalt cement. Excessive asphalt cement reduces the skid-resistance of a pavement, and it can become very slippery when wet, creating a safety hazard. This is caused by an excessively high asphalt cement content in the mix, using an asphalt cement with too low a viscosity (too flow able), too heavy a prime or tack coat, or an improperly applied seal coat. Bleeding occurs more often in hot weather when the asphalt cement is less viscous (more flow able) and the traffic forces the asphalt to the surface.

3. Polishing: Polishing is the wearing of aggregate on the pavement surface due to traffic. It can result in a dangerous low friction surface. A thin wearing course will repair the surface.

IV. CAUSES OF PAVEMENT DETERIORATION). (i)Sudden increase in traffic loading especially on new roads where the design is based on lesser traffic is a major cause of cracking. After construction of good road, traffic of other roads also shifts to that road. This accelerates the fatigue failure (Alligator Cracking(ii)Temperature variation ranging from 50 C to below zero conditions in the plain areas of North and Central India leads to bleeding and cracking. (iii) Provision of poor shoulders leads to edge failures.

(iv) Provision of poor clayey subgrade results in corrugation at the surface and increase in unevenness. . (v) Poor drainage conditions especially during rainy seasons, force the water to enter the pavement from the sides as well as from the top surface. In case of open graded bituminous layer, this phenomenon becomes more dangerous and the top layer gets detached from the lower layers. (vi) If the temperature of bitumen/bituminous mixes is not maintained properly, then it also leads to pavement failure. Over heating of bitumen reduces the binding property of bitumen. If the temperature of bituminous mix has been lowered down then the compaction will not be proper leading to longitudinal corrugations.

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